Switched capacitor filter having a reduced capacitance

Abstract

In a switched capacitor filter of a first-order responsive to a filter input signal for producing a filter output signal by carrying out sampling operation at a predetermined sampling rate by the use of switching circuit, a capacitor, and an integrating circuit, a voltage divider produces a voltage divided signal in response to the filter input signal to reduce a total capacitance determined by capacitances of the capacitor and the integrating circuit. The voltage divided signal is sampled by the switching circuit to be sent to the integrating circuit at the sampling rate through the capacitor. The integrating circuit produces the filter output signal as a result of integration. In addition, an additional switched capacitor filter of a first-order is connected in cascade to the filter to form a switched capacitor filter of a second-order. The additional filter comprises an additional voltage divider for voltage dividing an output signal of the additional filter to deliver an additional voltage divided signal to the filter and the additional filter.

What is claimed is: 
     
       1. A switched capacitor filter comprising voltage dividing means responsive to a filter input signal for producing a filter output signal V out , said filter having a transfer function V out  /V in  which is specified by a z-transform related to a unit capacitance C and which is given by:   V.sub.out /V.sub.in =1[αz.sup.1/2 (1-z.sup.-1)]     where α represents a capacitance ratio determined in relationship to said unit capacitance and z is equal to e j2 πf/fc and where fc represents a sampling frequency and f represents a frequency of said filter input signal, said filter comprising:   a first resistor having a first resistance of R 1 , a first resistor terminal supplied with said filter input signal, and a second resistor terminal;   a second resistor having a second resistance of R 2 , a third resistor terminal connected in common to said second resistor terminal, and a fourth resistor terminal to be grounded;   a first capacitor having a first and a second electrode and a first capacitance given by:   C(R.sub.1 +R.sub.2)/R.sub.2 ;       first switching means connected to said first and said second electrodes for switching said input signal to produce a switched signal at a sampling frequency;   an operational amplifier having an amplifier input signal supplied with sid switched signal and an amplifier output signal for said filter output signal; and   an integrating capacitor connected between said amplifier input terminal and said amplifier output terminal and having a capacitance equal to C.   
     
     
       2. A switched capacitor filter as claimed in claim 1, said filter output signal being produced as a first filter signal, said switched capacitor filter being for producing a second filter signal in response to said first filter signal, said switched capacitor filter further comprising: a second capacitor having a first and a second additional terminal;   second switching means connected to said first and said second additional electrodes for switching said first filter signal at said predetermined sampling period to produce an additional switched signal at said predetermined sampling period; and   second integrating means for integrating said additional switched signal to produce said second filter signal.   
     
     
       3. A switched capacitor filter as claimed in claim 2, further comprising: additional voltage dividing means connected to said second integrating means for voltage dividing said second filter signal into an additional voltage divided signal;   third switching means connected to said additional voltage dividing means for switching said additional voltage divided signal at said predetermined sampling period to produce an auxiliary switched signal; and   a third capacitor between said third and said second switching means for transmitting said auxiliary switched signal from said third switching means to said second switching means.   
     
     
       4. A switched capacitor filter as claimed in claim 3, further comprising: a fourth capacitor between said third and said first switching means.   
     
     
       5. A switched capacitor filter as claimed in claim 4, further comprising: a bypass capacitor between said second switching means and said first resistor terminal.